Many technologies use electromagnetic waves for communication, detection and ranging, offensive or defensive purposes, etc. Some of these technologies rely on or can be improved by determination of the direction of arrival of an electromagnetic wave. For example, some communication systems use a directional antenna, such as a parabolic dish, to provide directional control of a transmitted electromagnetic wave or to selectively receive an electromagnetic wave from a particular direction. Such antennas are referred to as “directional” because they are configured to focus electromagnetic waves along a pointing direction (corresponding, for example, to a narrow range of directions of arrival) of the antenna. Some directional antennas are repositionable to change the pointing direction of the directional antenna. Such repositionable directional antennas can be used to estimate a direction of arrival of an electromagnetic wave, since the direction of arrival of the electromagnetic wave corresponds to pointing direction of the repositionable directional antenna when the electromagnetic wave is received. As an example, for a directional antenna that uses a parabolic dish, the parabolic dish alone is generally sized to have a radius of between one quarter (¼) of a wavelength of a target electromagnetic wave and one (1) wavelength of a target electromagnetic wave. Further, a receiver or transmitter is positioned at a focal point of the parabolic dish to receive or transmit a signal. Further, if the directional antenna is repositionable, an actuator, gimbals, or other pointing support equipment may be coupled to the parabolic dish. Thus, a repositionable directional antenna system can be relatively large, heavy, and expensive.
Rather than physically repositioning a directional antenna, some systems use an array of radiating elements, which may be stationary or movable, and use beamforming techniques to electronically control a pointing direction of the array. In such systems, each radiating element is a small antenna (e.g., a conductor); therefore, each radiating element re-radiates some energy that it receives from an electromagnetic wave. Re-radiation from the radiating elements of the array is a source of noise that can limit a signal to noise ratio (SNR) or a detection limit of the array or nonlinearity signal distortions. Further, such arrays can be sized such that spacing between adjacent radiating elements is greater than one quarter (¼) of a wavelength of a target electromagnetic wave. Thus, arrays with many radiating elements can be relatively large and heavy. Also, beamforming techniques used for electronically pointing such arrays can be computationally complex and require that each radiating element have support hardware, such as a phase shifter and/or amplifier, to provide directional control.
Accordingly, while determining direction of arrival of electromagnetic waves is important for many technologies, available systems for determining direction of arrival tend to be large, heavy, complex, and/or expensive.